The use of the technique known as "thermal sizing" is well established in the fabrication of precisely dimensioned tubular bodies and in various other processes. This technique takes advantage of differences in the coefficients of thermal expansion of different metals. A mandrel having a coefficient of thermal expansion greater than that of the tubular body to be sized is used. An assembly of the tubular body and mandrel is heated and, as the mandrel expands at a greater rate than the tubular body, it plastically deforms the latter to the desired dimensions. The assembly is then cooled and the mandrel removed. The difficulty in this process arises from the fact that the difference between the thermal expansion of the mandrel and that of the tubular body is relatively small, i.e. on the order of 0.040 inches for a 5 inch diameter body and a desirable heating temperature. This means that the gap between the inner walls of the tubular body and the outer walls of the mandrel prior to heating must be considerably less than 0.040 inches. At some stage in the heat-up, the mandrel must be thermally expanded into contact with the tubular body for thermal sizing to be effective. Another problem arises upon assembly when the mandrel is slid into the tubular body. If there is the tight fit necessary for later expansion and if the size and straightness are outside acceptable limits of variation, it will either cause the mandrel to gall or mar the inner walls of the tubular body or prevent the mandrel from being forced in.
Attempts have been made to solve these problems in a number of different ways. One such method is described in U.S. Pat. No. 3,559,278. The tubular body is formed by bending a long flat strip of metal around a mandrel having a circular cross-section. The longitudinal edge surfaces of the metal strip are butt welded together. The tubular body which is thus formed is in close contact with the mandrel. Next, the assembly is heated to a predetermined temperature. The selected temperature is a function of the mandrel size, the desired final dimensions of the tubular body, and the difference between the coefficients of thermal expansion of the mandrel and the tube. Further, the selected temperature must be high enough for the mandrel to create sufficient lateral stress to expand the tube so that, upon cooling, the tube has the required lateral, i.e. cross-sectional, dimensions. This method of welding the tube around the mandrel obviates the need for sliding the mandrel into the tubular body, but adds complexity to the welding steps since welding must be performed while the mandrel is inside the tubular body.
Another method for assembling the mandrel and tubular body is described in U.S. Pat. No. 3,986,654. An approximately sized but somewhat oversized tubular body is mounted on a mandrel having a larger coefficient of thermal expansion than the tubular body. The ends of the mandrel are attached to the ends of the tubular body. On heating, the tubular body is first stressed longitudinally by the lengthening of the mandrel. This action causes the inner walls of the tubular body to contract in a lateral direction so as to contact the outer walls of the mandrel. Subsequently, as the mandrel expands laterally at a greater rate than the tubular body, the latter is expanded in both lateral and longitudinal directions. This method avoids the need for a tight fit between the tubular body and the mandrel and hence it obviates the problem of marring the tubular body during assembly.
Such methods as described above have made possible the fabrication of elongated hollow bodies to closer tolerances than had been previously possible. Nevertheless, certain problems still persist. Forming the tubular body around the mandrel with consistent, continuous, surface-to-surface contact has been found to be difficult with particular metals, e.g. metals used to make nuclear fuel channels. Further, the need to attach the ends of the mandrel and tubular body creates an added degree of complexity in the fabrication process. Also, the longitudinal stretching may adversely affect the wall thickness of the tubular body. Additionally, if the tubular body and mandrel are not in surface-to-surface contact at the beginning of the heating step, then full advantage is not taken of the lateral expansion of the mandrel. As a consequence, the temperature to which the assembly must be heated is higher, thus making the process less efficient.